1. Field of the Invention
The present invention relates to display screens comprising a layer of electro-optical material, for example a layer of liquid crystal.
2. Description of the Prior Art
As is known, these screens generally comprise a large number of square or rectangular pixels. The definition of the screen depends on the number of pixels capable of receiving an information element. Each pixel is controlled by the application of an electrical field. For the display of video information elements, matrix type displays have been proposed. Each pixel is then defined by the intersection of two arrays of orthogonal conductors called lines and columns. It then becomes possible to reduce the number of connections of M elements from a value M to twice the square root of M. By contrast, the matrix structure raises serious problems as regards the control of the screen and of the electro-optical material having the task of converting the electrical signal into a visual signal. Indeed, in this type of addressing, it is possible to simultaneously address only the pixels of one and the same line i in applying, to this line, an exciting voltage Vi and in applying, to the columns, voltages Vj that depend on the state which is to be assumed by the pixels (i,j). Each line of the screen is excited sequentially during a period t = T/N where T is the screen addressing time and N is the number of lines. If the electro-optical material does not have any memory of its own, the information has to be maintained by the periodic re-excitation of the activated pixels. The maximum value of this regeneration period T is fixed physiologically at about 40 ms. Hence, the time T devoted to the excitation of a pixel is t = 40/N ms and becomes very short when N increases.
One of the approaches envisaged to overcome this problem is to use screens with active control for which the electro-optical material is placed in series with an electronic switch (such as a diode or a transistor). The picture memory may thus be constituted by two arrays of line and column conductors, at the intersection of which there is, for example, a switching transistor connected to the capacitor formed by the electro-optical material and its control electrodes (FIG. 1). In this case, the gate of the transistor is connected to the line electrode and its source to the column electrode, through which there flows the video frequency signal. When a line i is excited, a voltage is applied so as to make all the transistors of the line conductive. The capacitors of the electro-optical cells get charged at the video voltages applied to the columns. When the line i stops being excited, the transistors are off, and the information elements are kept in the capacitors of the previously addressed line.
The improvement of the performance characteristics of a TFT (thin film transistor) screen, as compared with a screen with direct addressing, is obtained however at the cost of increased technological complexity due to the number of vacuum depositions and of masking steps needed for the making of the transistors of the screen, as well as to faults such as cut lines, line-column short-circuits or defective pixels. The short-circuits between lines and columns arise chiefly out of the fact that the lines and columns are formed on one and the same glass plate.